Process and apparatus for obtaining bulk monocrystalline gallium-containing nitride

ABSTRACT

The present invention refers to an ammonobasic method for preparing a gallium-containing nitride crystal, in which gallium-containing feedstock is crystallized on at least one crystallization seed in the presence of an alkali metal-containing component in a supercritical nitrogen-containing solvent. The method can provide monocrystalline gallium-containing nitride crystals having a very high quality.

[0001] The present invention refers to processes for obtaining agallium-containing nitride crystal by an ammonobasic method as well asthe gallium-containing nitride crystal itself. Furthermore, an apparatusfor conducting the various methods is disclosed.

[0002] Optoelectronic devices based on nitrides are usually manufacturedon sapphire or silicon carbide substrates that differ from the depositednitride layers (so-called heteroepitaxy). In the most often usedMetallo-Organic Chemical Vapor Deposition (MOCVD) method, the depositionof GaN is performed from ammonia and organometallic compounds in the gasphase and the growth rates achieved make it impossible to provide a bulklayer. The application of a buffer layer reduces the dislocationdensity, but not more than to approx. 10⁸/cm². Another method has alsobeen proposed for obtaining bulk monocrystalline gallium nitride. Thismethod consists of an epitaxial deposition employing halides in a vaporphase and is called Halide Vapor Phase Epitaxy (HVPE) [“Opticalpatterning of GaN films” M. K. Kelly, O. Ambacher, Appl. Phys. Lett. 69(12) (1996) and “Fabrication of thin-film InGaN light-emitting diodemembranes” W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)].This method allows for the preparation of GaN substrates having a 2-inch(5 cm) diameter.

[0003] However, their quality is not sufficient for laser diodes,because the dislocation density continues to be approx. 10⁷ to approx.10⁹/cm². Recently, the method of Epitaxial Lateral OverGrowth (ELOG) hasbeen used for reducing the dislocation density. In this method the GaNlayer is first grown on a sapphire substrate and then a layer with SiO₂is deposited on it in the form of strips or a lattice. On the thusprepared substrate, in turn, the lateral growth of GaN may be carriedout leading to a dislocation density of approx. 10⁷/cm².

[0004] The growth of bulk crystals of gallium nitride and other metalsof group XIII (IUPAC, 1989) is extremely difficult. Standard methods ofcrystallization from melt and sublimation methods are not applicablebecause of the decomposition of the nitrides into metals and N₂. In theHigh Nitrogen Pressure (HNP) method [“Prospects for high-pressurecrystal growth of III-V nitrides” S. Porowski et al., Inst. Phys. Conf.Series, 137, 369 (1998)] this decomposition is inhibited by the use ofnitrogen under the high pressure. The growth of crystals is carried outin molten gallium, i.e. in the liquid phase, resulting in the productionof GaN platelets about 10 mm in size. Sufficient solubility of nitrogenin gallium requires temperatures of about 1500° C. and nitrogenpressures in the order of 15 kbar.

[0005] The use of supercritical ammonia has been proposed to lower thetemperature and decrease the pressure during the growth process ofnitrides. Peters has described the ammonothermal synthesis of aluminiumnitride [J. Cryst. Growth 104, 411-418 (1990)]. R. Dwilinski et al. haveshown, in particular, that it is possible to obtain a fine-crystallinegallium nitride by a synthesis from gallium and ammonia, provided thatthe latter contains alkali metal amides (KNH₂ or LiNH₂). The processeswere conducted at temperatures of up to 550° C. and under a pressure of5 kbar, yielding crystals about 5 μm in size [“AMMONO method of BN, AlN,and GaN synthesis and crystal growth”, Proc. EGW-3, Warsaw, Jun. 22-24,1998, MRS Internet Journal of Nitride Semiconductor Research,http://nsr.mij.mrs.org/3/25]. Another supercritical ammonia method,where a fine-crystalline GaN is used as a feedstock together with amineralizer consisting of an amide (KNH₂) and a halide (KI) alsoprovided for recrystallization of gallium nitride [“Crystal growth ofgallium nitride in supercritical ammonia” J. W. Kolis et al., J. Cryst.Growth 222, 431-434 (2001)]. The recrystallization process conducted at400° C. and 3.4 kbar resulted in GaN crystals about 0.5 mm in size. Asimilar method has also been described in Mat. Res. Soc. Symp. Proc.Vol. 495, 367-372 (1998) by J. W. Kolis et al. However, using thesesupercritical ammonia processes, no production of bulk monocrystallinewas achieved because no chemical transport processes were observed inthe supercritical solution, in particular no growth on seeds wasconducted.

[0006] Therefore, there was a need for an improved method of preparing agallium-containing nitride crystal.

[0007] The lifetime of optical semiconductor devices depends primarilyon the crystalline quality of the optically active layers, andespecially on the surface dislocation density. In case of GaN basedlaser diodes, it is beneficial to lower the dislocation density in theGaN substrate layer to less than 10⁶/cm², and this has been extremelydifficult to achieve using the methods known so far. Therefore, therewas a need for gallium-containing nitride crystals having a qualitysuitable for use as substrates for optoelectronics.

[0008] The subject matter of the present invention is recited in theappended claims. In particular, in one embodiment the present inventionrefers to a process for obtaining a gallium-containing nitride crystal,comprising the steps of:

[0009] (i) providing a gallium-containing feedstock, an alkalimetal-containing component; at least one crystallization seed and anitrogen-containing solvent in at least one container;

[0010] (ii) bringing the nitrogen-containing solvent into asupercritical state;

[0011] (iii) at least partially dissolving the gallium-containingfeedstock at a first temperature and at a first pressure; and

[0012] (iv) crystallizing gallium-containing nitride on thecrystallization seed at a second temperature and at a second pressurewhile the nitrogen-containing solvent is in the supercritical state;

[0013] wherein at least one of the following criteria is fulfilled:

[0014] (a) the second temperature is higher than the first temperature;and

[0015] (b) the second pressure is lower than the first pressure.

[0016] In a second embodiment a process for preparing agallium-containing nitride crystal is described which comprises thesteps of:

[0017] (i) providing a gallium-containing feedstock comprising at leasttwo different components, an alkali metal-containing component, at leastone crystallization seed and a nitrogen-containing solvent in acontainer having a dissolution zone and a crystallization zone, wherebythe gallium-containing feedstock is provided in the dissolution zone andthe at least one crystallization seed is provided in the crystallizationzone;

[0018] (ii) subsequently bringing the nitrogen-containing solvent into asupercritical state;

[0019] (iii) subsequently partially dissolving the gallium-containingfeedstock at a dissolution temperature and at a dissolution pressure inthe dissolution zone, whereby a first component of thegallium-containing feedstock is substantially completely dissolved and asecond component of the gallium-containing feedstock as well as thecrystallization seed remain substantially undissolved so that anundersaturated or saturated solution with respect to gallium-containingnitride is obtained;

[0020] (iv) subsequently setting the conditions in the crystallizationzone at a second temperature and at a second pressure so thatover-saturation with respect to gallium-containing nitride is obtainedand crystallization of gallium-containing nitride occurs on the at leastone crystallization seed and simultaneously setting the conditions inthe dissolution zone at a first temperature and at a first pressure sothat the second component of the gallium-containing feedstock isdissolved;

[0021] wherein the second temperature is higher than the firsttemperature.

[0022] A gallium-containing nitride crystal obtainable by one of theseprocesses is also described. Further subject matter of the invention area gallium-containing nitride crystal having a surface area of more than2 cm² and having a dislocation density of less than 10^(6/cm) ² and agallium-containing nitride crystal having a thickness of at least 200 μmand a full width at half maximum (FWHM) of X-ray rocking curve from(0002) plane of 50 arcsec or less.

[0023] The invention also provides an apparatus for obtaining agallium-containing nitride crystal comprising an autoclave 1 having aninternal space and comprising at least one device 4, 5 for heating theautoclave to at least two zones having different temperatures, whereinthe autoclave comprises a device which separates the internal space intoa dissolution zone 13 and a crystallization zone 14.

[0024] In a yet another embodiment, a process for preparing a bulkmonocrystalline gallium-containing nitride in an autoclave is disclosed,which comprises the steps of providing a supercritical ammonia solutioncontaining gallium-containing nitride with ions of alkali metals, andrecrystallizing said gallium-containing nitride selectively on acrystallization seed from said supercritical ammonia solution by meansof the negative temperature coefficient of solubility and/or by means ofthe positive pressure coefficient of solubility.

[0025] A process for controlling recrystallization of agallium-containing nitride in a supercritical ammonia solution whichcomprises steps of providing a supercritical ammonia solution containinga gallium-containing nitride as a gallium complex with ions of alkalimetal and NH₃ solvent in an autoclave and decreasing the solubility ofsaid gallium-containing nitride in the supercritical ammonia solution ata temperature less than that of dissolving gallium-containing nitridecrystal and/or at a pressure higher than that of dissolvinggallium-containing nitride crystal is also disclosed.

[0026]FIG. 1 shows the dependency of the solubility ofgallium-containing nitride in supercritical ammonia that containspotassium amide (with KNH₂:NH₃=0.07) on pressure at T=400° C. and T=500°C.

[0027]FIG. 2 shows the diagram of time variations of temperature in anautoclave at constant pressure for Example 1.

[0028]FIG. 3 shows the diagram of time variations of pressure in anautoclave at constant temperature for Example 2.

[0029]FIG. 4 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 3.

[0030]FIG. 5 shows the diagram of time variations of temperature in anautoclave for Example 4.

[0031]FIG. 6 shows the diagram of time variations of temperature in anautoclave for Example 5.

[0032]FIG. 7 shows the diagram of time variations of temperature in anautoclave for Example 6.

[0033]FIG. 8 shows the diagram of time variations of temperature in anautoclave for Example 7.

[0034]FIG. 9 shows a schematic axial cross section of an autoclave asemployed in many of the examples, mounted in the furnace.

[0035]FIG. 10 is a schematic perspective drawing of an apparatusaccording to the present invention.

[0036]FIG. 11 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 8.

[0037]FIG. 12 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 9.

[0038]FIG. 13 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 10.

[0039]FIG. 14 shows the diagram of time variations of temperature in anautoclave at constant volume for Examples 11 and 12.

[0040]FIG. 15 illustrates the postulated theory of the invention.

[0041] In the present invention the following definitions apply:

[0042] Gallium-containing nitride means a nitride of gallium andoptionally other element(s) of group XII (according to IUPAC, 1989). Itincludes, but is not restricted to, the binary compound GaN, ternarycompounds such as AlGaN, InGaN and also AlInGaN (The mentioned formulasare only intended to give the components of the nitrides. It is notintended to indicate their relative amounts).

[0043] Bulk monocrystalline gallium-containing nitride means amonocrystalline substrate made of gallium-containing nitride from whiche.g. optoelectronic devices such as LED or LD can be formed by epitaxialmethods such as MOCVD and HVPE.

[0044] Supercritical solvent means a fluid in a supercritical state. Itcan also contain other components in addition to the solvent itself aslong as these components do not substantially influence or disturb thefunction of the supercritical solvent. In particular, the solvent cancontain ions of alkali metals.

[0045] Supercritical solution is used when referring to thesupercritical solvent when it contains gallium in a soluble formoriginating from the dissolution of gallium-containing feedstock.

[0046] Dissolution of gallium-containing feedstock means a process(either reversible or irreversible) in which said feedstock is taken upinto the supercritical solvent as gallium in a soluble form, possibly asgallium complex compounds.

[0047] Gallium complex compounds are complex compounds, in which agallium atom is a coordination center surrounded by ligands, such as NH₃molecules or its derivatives, like NH₂ ⁻, NH²⁻, etc.

[0048] Negative temperature coefficient of solubility means that thesolubility of a respective compound is a monotonically decreasingfunction of temperature if all other parameters are kept constant.Similarly, positive pressure coefficient of solubility means that, ifall other parameters are kept constant, the solubility is amonotonically increasing function of pressure. In our research we showedthat the solubility of gallium-containing nitride in supercriticalnitrogen-containing solvents, such as ammonia possesses a negativetemperature coefficient and a positive pressure coefficient intemperatures ranging at least from 300 to 600° C. and pressures from 1to 5.5 kbar.

[0049] Over-saturation of supercritical solution with respect togallium-containing nitride means that the concentration of gallium in asoluble form in said solution is higher than that in equilibrium (i.e.it is higher than solubility). In the case of dissolution ofgallium-containing nitride in a closed system, such an over-saturationcan be achieved by either increasing the temperature and/or decreasingthe pressure.

[0050] Spontaneous crystallization means an undesired process wherenucleation and growth of the gallium-containing nitride fromover-saturated supercritical solution take place at any site within anautoclave except at the surface of a crystallization seed where thegrowth is desired. Spontaneous crystallization also comprises nucleationand disoriented growth on the surface of crystallization seed.

[0051] Selective crystallization on a seed means a process ofcrystallization on a seed carried out without spontaneouscrystallization.

[0052] Autoclave means a closed container which has a reaction chamberwhere the ammonobasic process according to the present invention iscarried out.

[0053] The present invention can provide a gallium-containing nitridemonocrystal having a large size and a high quality. Suchgallium-containing nitride crystals can have a surface area of more than2 cm² and a dislocation density of less than 10⁶/cm². Gallium-containingnitride crystals having a thickness of at least 200 μm (preferably atleast 500 μm) and a FWHM of 50 arcsec or less can also be obtained.Depending on the crystallization conditions, it possible to obtaingallium-containing nitride crystals having a volume of more than 0.05cm³, preferably more than 0.1 cm³ using the processes of the invention.

[0054] As was explained above, the gallium-containing nitride crystal isa crystal of nitride of gallium and optionally other element(s) of GroupXIII (the numbering of the groups is given according to the IUPACconvention of 1989 throughout this application). These compounds can berepresented by the formula Al_(x)Ga_(1-x-y)In_(y)N, wherein 0≦x<1,0≦y<1, 0≦x+y<1 (preferably 0≦x<0.5 and 0≦y<0.5). Although in a preferredembodiment, the gallium-containing nitride is gallium nitride, in afarther preferred embodiment part (e.g. up to 50 mol.-%) of the galliumatoms can be replaced by one or more other elements of Group XIII(especially Al and/or In).

[0055] The gallium-containing nitride may additionally include at leastone donor and/or at least one acceptor and/or at least one magneticdopant e.g. to alter the optical, electrical and magnetic properties ofthe substrate. Donor dopants, acceptor dopants and magnetic dopants arewell-known in the art and can be selected according to the desiredproperties of the substrate. Preferably the donor dopants are selectedfrom the group consisting of Si and O. As acceptor donors Mg and Zn arepreferred. Any known magnetic dopant can be included into the substratesof the present invention. A preferred magnetic dopant is Mn and possiblyalso Ni and Cr. The concentrations of the dopants are well-known in theart and depend on the desired end application of the nitride. Typicallythe concentrations of these dopants range from 10¹⁷ to 10²¹/cm³. Insteadof adding dopants as part of the feedstock into the autoclave, dopantscan also be included into the gallium-containing nitride crystal fromtrace amounts of the autoclave material which dissolve during theprocess of the invention. For example, if the autoclave comprises anickel alloy then nickel can be included into the gallium-containingnitride crystal.

[0056] Due to the preparation process the gallium-containing nitridecrystal can also contain alkali elements, usually in an amount of morethan about 0.1 ppm. Generally it is desired to keep the alkali elementscontent lower than 10 ppm, although it is difficult to specify whatconcentration of alkali metals in gallium-containing nitride has adisadvantageous influence on its properties.

[0057] It is also possible that halogens are present in thegallium-containing nitride. The halogens can be introduced eitherintentionally (as a component of the mineralizer) or unintentionally(from impurities of the mineralizer or the feedstock). It is usuallydesired to keep the halogen content of the gallium-containing nitridecrystal in the range of about 0.1 ppm or less.

[0058] The process of the invention is a supercritical crystallizationprocess, which includes at least two steps: a dissolution step at afirst temperature and at a first pressure and a crystallization step ata second temperature and at a second pressure. Since generally highpressures and/or high temperatures are involved, the process accordingto the invention is preferably conducted in an autoclave. The two steps(i.e. the dissolution step and the crystallization step) can either beconducted separately or can be conducted at least partiallysimultaneously in the same reactor.

[0059] For conducting the two steps separately, the process can beconducted in one single reactor but the dissolution step is conductedbefore the crystallization step. In this embodiment the reactor can havethe conventional construction of a single chamber. The process of theinvention in the two-step embodiment can be conducted using constantpressure and two different temperatures or using constant temperatureand two different pressures. It is also possible to use two differentpressures and two different temperatures. The exact values of pressureand temperature should be selected depending on the feedstock, thespecific nitride to be prepared and the solvent. Generally the pressureis in the range of 1 to 10 kbar, preferably 1 to 5.5 kbar and morepreferably 1.5 to 3 kbar. The temperature is usually in the range of100° C. to 800° C., preferably 300° C. to 600° C., more preferably 400°C. to 550° C. If two different pressures are employed, the difference inpressure should be from 0.1 kbar to 9 kbar, preferably from 0.2 kbar to3 kbar. However, if the dissolution and crystallization are controlledby the temperature, the difference in temperature should be at least 1°C., and preferably from 5° C. to 150° C.

[0060] In a preferred embodiment, the dissolution step and thecrystallization step are conducted at least partially simultaneously inthe same container. For such an embodiment the pressure is practicallyuniform within the container, while the temperature difference betweenthe dissolution zone and crystallization zone should be at least 1° C.,and preferably is from 5° C. to 150° C. Furthermore, the temperaturedifference between the dissolution zone and crystallization zone shouldbe controlled so as to ensure chemical transport in the supercriticalsolution, which takes place through convection.

[0061] A possible construction of a preferred container is given in FIG.9. For conciseness and ease of understanding in the following, theprocess will be explained particularly with respect to this preferredembodiment. However, the invention can be conducted with differentcontainer constructions as long as the principles outlined in thespecification and the claims are adhered to.

[0062] In a preferred embodiment of the invention, the process can beconducted in an apparatus comprising an autoclave 1 having an internalspace and comprising at least one device 4, 5 for heating the autoclaveto at least two zones having different temperatures, wherein theautoclave comprises a device which separates the internal space into adissolution zone 13 and a crystallization zone 14 (hereinafter alsoreferred to as “separating device” or “installation”). These two zoneshaving different temperatures should preferably coincide with thedissolution zone 13 and the crystallization zone 14. The device whichseparates the internal space of the autoclave can be, for example, atleast one baffle 12 having at least one opening 2. Examples are baffleshaving a central opening, circumferential openings or a combinationthereof. The size of the opening(s) 2 should be large enough to allowtransport between the zones but should be sufficiently small to maintaina temperature gradient in the reactor. The appropriate size of theopening(s) depends on the size and the construction of the reactor andcan be easily determined by a person skilled in the art.

[0063] In one embodiment, two different heating devices can be employed,the position of which preferably corresponds to the dissolution zone 13and the crystallization zone 14. However, it has been observed thattransport of gallium in a soluble form from the dissolution zone 13 tothe crystallization zone 14 can be further improved if a cooling means 6is present between the first and the second heating devices and islocated at approximately the position of the separating device. Thecooling means 6 can be realized by liquid (e.g. water) cooling orpreferably by fan cooling. The heating devices can be poweredelectrically, by either inductive or, preferably, by resistive heatingmeans. Use of a heating-cooling-heating configuration gives widerpossibilities in forming the desired temperature distribution within theautoclave. For example, it enables to obtain a low temperature gradientswith in most of the crystallization zone 14 and a low temperaturegradient within most of the dissolution zone 13, while achieving a hightemperature gradient in the region of the baffle 12.

[0064] When the process of the present, invention is conducted agallium-containing feedstock, an alkali metal-containing component, atleast one crystallization seed and a nitrogen-containing solvent areprovided in at least one container. In the preferred apparatus describedabove, the gallium-containing feedstock 16 is placed in the dissolutionzone 13 and the at least one crystallization seed 17 is placed in thecrystallization zone 14. The alkali metal-containing component is alsopreferably placed in the dissolution zone. Then the nitrogen-containingsolvent is added to the container, which is then closed. Subsequentlythe nitrogen-containing solvent is brought into a supercritical state,e.g. by increasing pressure and/or heat.

[0065] In the present invention any materials containing gallium, whichare soluble in the supercritical solvent under the conditions of thepresent invention, can be used as a gallium-containing feedstock.Typically the gallium-containing feedstock will be a substance ormixture of substances, which contains at least gallium, and optionallyalkali metals, other Group XIII elements, nitrogen, and/or hydrogen,such as metallic Ga, alloys and inter-metallic compounds, hydrides,amides, imides, amido-imides, azides. Suitable gallium-containingfeedstocks can be selected from the group consisting of gallium nitrideGaN, azides such as Ga(N₃)₃, imides such as Ga₂(NH)₃, amido-imides suchas Ga(NH)NH₂, amides such as Ga(NH₂)₃, hydrides such as GaH₃,gallium-containing alloys, metallic gallium and mixtures thereof.Preferred feedstocks are metallic gallium and gallium nitride andmixtures thereof. Most preferably, the feedstock is metallic gallium andgallium nitride. If elements of Group XIII other than gallium are to bepresent in the gallium-containing nitride crystal, correspondingcompounds or mixed compounds including Ga and the other Group XIIIelement can be used. If the substrate is to contain dopants or otheradditives, precursors thereof can be added to the feedstock.

[0066] The form of the feedstock is not particularly limited and it canbe in the form of one or more pieces or in the form of a powder. If thefeedstock is in the form of a powder, care should be taken thatindividual powder particles are not transported from the dissolutionzone to the crystallization zone, where they can cause irregularcrystallization. It is preferable that the feedstock is in one or morepieces and that the surface area of the feedstock is larger than that ofthe crystallization seed.

[0067] The nitrogen-containing solvent employed in the present inventionmust be able to form a supercritical fluid, in which gallium can bedissolved in the presence of alkali metal ions. Preferably the solventis ammonia, a derivative thereof or mixtures thereof. An example of asuitable ammonia derivative is hydrazine. Most preferably the solvent isammonia. To reduce corrosion of the reactor and to avoid side-reactions,halogens e.g. in the form of halides are preferably not intentionallyadded into the container. Although traces of halogens may be introducedinto the system in the form of unavoidable impurities of the startingmaterials, care should be taken to keep the amount of halogen as low aspossible. Due to the use of a nitrogen-containing solvent such asammonia it is not necessary to include nitride compounds into thefeedstock. Metallic gallium (or aluminium or indium) can be employed asthe feedstock while the solvent provides the nitrogen required for thenitride formation.

[0068] It has been observed that the solubility of gallium-containingfeedstock, such as gallium and corresponding elements of Group XIIIand/or their compounds, can be significantly improved by the presence ofat least one type of alkali metal-containing component as asolubilization aid (“mineralizer”). Lithium, sodium and potassium arepreferred as alkali metals, wherein sodium and potassium are morepreferred. The mineralizer can be added to the supercritical solvent inelemental form or preferably in the form of its compound (such as asalt). Generally the choice of the mineralizer depends on the solventemployed in the process. According to our investigations, alkali metalhaving a smaller ion radius can provide lower solubility ofgallium-containing nitride in the supercritical solvent than thatobtained with alkali metals having a larger ion radius. For example, ifthe mineralizer is in the form of a compound such as a salt, it ispreferably in the form of an alkali metal hydride such as MH, an alkalimetal nitride such as M₃N, an alkali metal amide such as MNH₂, an alkalimetal imide such as M₂NH or an alkali metal azide such as MN₃ (wherein Mis an alkali metal). The concentration of the mineralizer is notparticularly restricted and is selected so as to ensure adequate levelsof solubility of both feedstock (the starting material) andgallium-containing nitride (the resulting product). It is usually in therange of 1:200 to 1:2, in the terms of the mols of the metal ion basedon the mols of the solvent (molar ratio). In a preferred embodiment theconcentration is from 1:100 to 1:5, more preferably 1:20 to 1:8 mols ofthe metal ion based on the mols of the solvent.

[0069] The presence of the alkali metal ions in the process can lead toalkali metal in the thus prepared substrates. It is possible that theamount of alkali metal is more than about 0.1 ppm, even more than 10ppm. However, in these amounts the alkali metals do not detrimentallyeffect the properties of the substrates. It has been found that even atan alkali metal content of 500 ppm, the operational parameters of thesubstrate according to the invention are still satisfactory.

[0070] The dissolved feedstock crystallizes in the crystallization stepunder the low solubility conditions on the crystallization seed(s) whichare provided in the container. The process of the invention allows bulkgrowth of monocrystalline gallium-containing nitride on thecrystallization seed(s) and in particular leads to the formation ofstoichiometric gallium-containing nitride in the form of a bulkmonocrystalline layer on the crystallization seed(s).

[0071] Various crystals can be used as crystallization seeds in thepresent invention. however, it is preferred that the chemical andcrystallographic constitution of the crystallization seeds is similar tothose of the desired layer of bulk monocrystalline gallium-containingnitride. Therefore, the crystallization seed preferably comprises acrystalline layer of gallium-containing nitride. To facilitatecrystallization of the dissolved feedstock, the dislocation density ofthe crystallization seed is preferably less than 10⁶/cm². Suitablecrystallization seeds generally have a surface area of 8×8 mm² or moreand thickness of 100 μm or more, and can be obtained e.g. by HVPE.

[0072] After the starting materials have been introduced into thecontainer and the nitrogen-containing solvent has been brought into itssupercritical state, the gallium-containing feedstock is at leastpartially dissolved at a first temperature and a first pressure, e.g. inthe dissolution zone of an autoclave. Gallium-containing nitridecrystallizes on the crystallization seed (e.g. in the crystallizationzone of an autoclave) at a second temperature and at a second pressurewhile the nitrogen-containing solvent is in the supercritical state,wherein the second temperature is higher than the first temperatureand/or the second pressure is lower than the first pressure. If thedissolution and the crystallization steps take place simultaneously inthe same container, the second pressure is essentially equal to thefirst pressure.

[0073] This is possible since the solubility of gallium-containingnitride under the conditions of the present invention shows a negativetemperature coefficient and a positive pressure coefficient in thepresence of alkali metal ions. Without wishing to be bound by theory, itis postulated that the following processes occur. In the dissolutionzone, the temperature and pressure are selected such that thegallium-containing feedstock is dissolved and the nitrogen-containingsolution is undersaturated with respect to gallium-containing nitride.In the crystallization zone, the temperature and pressure are selectedsuch that the solution, although it contains approximately the sameconcentration of gallium as in the dissolution zone, is over-saturatedwith respect to gallium-containing nitride. Therefore, crystallizationof gallium-containing nitride on the crystallization seed occurs. Thisis illustrated in FIG. 15. Due e.g. to the temperature gradient,pressure gradient, concentration gradient, different chemical orphysical character of dissolved feedstock and crystallized product etc.,gallium is transported in a soluble form from the dissolution zone tothe crystallization zone. In the present invention this is referred toas “chemical transport” of gallium-containing nitride in thesupercritical solution. It is postulated that the soluble form ofgallium is a gallium complex compound with a Ga atom in the coordinationcenter surrounded by ligands, such as NH₃ molecules or its derivatives,like NH₂ ⁻, NH²⁻, etc.

[0074] This theory is equally applicable for all gallium-containingnitrides, such as AlGaN. InGaN and AlInGaN as well as GaN (The mentionedformulas are only intended to give the components of the nitrides. It isnot intended to indicate their relative amounts). In the case ofnitrides other than gallium nitride aluminum and/or indium in a solubleform also have to be present in the supercritical solution.

[0075] In a preferred embodiment of the invention, thegallium-containing feedstock is dissolved in at least two steps. In thisembodiment, the gallium-containing feedstock generally comprises twokinds of starting materials which differ in solubility. The differencein solubility can be achieved chemically (e.g. by selecting twodifferent chemical compounds) or physically (e.g. by selecting two formsof the same compound having for example different surface areas, likemicrocrystalline powder and large crystals). In a preferred embodiment,the gallium-containing feedstock comprises two different chemicalcompounds such as metallic gallium and gallium nitride which dissolve atdifferent rates. In a first dissolution step, the first component of thegallium-containing feedstock is substantially completely dissolved at adissolution temperature and at a dissolution pressure in the dissolutionzone. The dissolution temperature and the dissolution pressure, whichcan be set only in the dissolution zone or preferably in the wholecontainer, are selected so that the second component of thegallium-containing feedstock and the crystallization seed(s) remainsubstantially undissolved. This first dissolution step results in anundersaturated or at most saturated solution (preferably undersaturatedsolution) with respect to gallium-containing nitride. For example, thedissolution temperature can be 100° C. to 350° C., preferably from 150°C. to 300° C. The dissolution pressure can be 0.1 kbar to 5 kbar,preferably from 0.1 kbar to 3 kbar.

[0076] Subsequently the conditions in the crystallization zone are setat a second temperature and at a second pressure so that over-saturationwith respect to gallium-containing nitride is obtained andcrystallization of gallium-containing nitride occurs on the at least onecrystallization seed. Simultaneously the conditions in the dissolutionzone are set at a first temperature and at a first pressure (preferablyequal to the second pressure) so that the second component of thegallium-containing feedstock is now dissolved (second dissolution step).As explained above the second temperature is higher than the firsttemperature and/or the second pressure is lower than the first pressureso that the crystallization can take advantage of the negativetemperature coefficient of solubility and/or of the positive pressurecoefficient of solubility. Preferably the first temperature is higherthan the dissolution temperature. During the second dissolution step andthe crystallization step, the system should be in a stationary state sothat the concentration of gallium in the supercritical solution remainssubstantially constant, i.e. approximately the same amount of galliumshould be dissolved per unit of time as is crystallized in the same unitof time. This allows for the growth of gallium-containing nitridecrystals of especially high quality and large size.

[0077] Typical pressures for the crystallization step and the seconddissolution step are in the range of 1 to 10 kbar, preferably 1 to 5.5kbar and more preferably 1.5 to 3 kbar. The temperature is generally inthe range of 100 to 800° C., preferably 300 to 600° C., more preferably400 to 550° C. The difference in temperature should be at least 1° C.,and is preferably from 5° C. to 150° C. As explained above, thetemperature difference between the dissolution zone and crystallizationzone should be controlled so as to ensure chemical transport in thesupercritical solution, which takes place through convection.

[0078] In the process of the invention, the crystallization should takeplace selectively on the crystallization seed and not on a wall of thecontainer. Therefore, the over-saturation extent with respect to thegallium-containing nitride in the supercritical solution in thecrystallization zone should be controlled so as to be below thespontaneous crystallization level where crystallization takes place on awall of the autoclave and/or disoriented growth occurs on the seed, i.e.the level at which spontaneous crystallization occurs. This can beachieved by adjusting the chemical transport rate and/or thecrystallization temperature and/or crystallization pressure. Thechemical transport is related on the speed of a convective flow from thedissolution zone to the crystallization zone, which can be controlled bythe temperature difference between the dissolution zone and thecrystallization zone, the size of the opening(s) of baffle(s) betweenthe dissolution zone and the crystallization zone etc.

[0079] The performed tests showed that the best bulk monocrystallinegallium nitride obtained had a dislocation density close to 10⁴/cm² andsimultaneously a FWHM of X-ray rocking curve from (0002) plane below 60arcsec. These crystals possess an appropriate quality and durability foroptical semiconductor devices. The gallium-containing nitride of thepresent invention typically has a wurzite structure.

[0080] Feedstock material for use in the present invention can also beprepared using a method similar to those described above. The methodinvolves the steps of:

[0081] (i) providing a gallium-containing feedstock, an alkalimetal-containing component, at least one crystallization seed and anitrogen-containing solvent in a container having at least one zone;

[0082] (ii) subsequently bringing the nitrogen-containing solvent into asupercritical state;

[0083] (iii) subsequently dissolving the gallium-containing feedstock(such as metallic gallium or aluminium or indium, preferably metallicgallium) at a dissolution temperature and at a dissolution pressure,whereby the gallium-containing feedstock is substantially completelydissolved and the crystallization seed remains substantially undissolvedso that an undersaturated solution with respect to gallium-containingnitride is obtained;

[0084] (iv) subsequently setting the conditions in at least part of thecontainer at a second temperature and at a second pressure so thatover-saturation with respect to gallium-containing nitride is obtainedand crystallization of gallium-containing nitride occurs on the at leastone crystallization seed;

[0085] wherein the second temperature is higher than the dissolutiontemperature.

[0086] In this embodiment the comments given above with respect to theindividual components, process parameters, etc. also apply. Preferablyduring the crystallization step in this embodiment the conditions in thewhole container are set at the second temperature and the secondpressure.

[0087] Gallium-containing nitride exhibits good solubility in asupercritical nitrogen-containing solvent (e.g. ammonia), providedalkali metals or their compounds, such as KNH₂, are introduced into it.FIG. 1 shows the solubility of gallium-containing nitride in asupercritical solvent versus pressure for temperatures of 400 and 500°C. wherein the solubility is defined by the molar percentage:S_(m)≡GaN^(solvent):(KNH₂+NH₃) 100%. In the present case the solvent issupercritical ammonia containing KNH₂ in a molar ratio x≡KNH₂:NH₃ equalto 0.07. For this case S_(m) should be a smooth function of only threeparameters: temperature, pressure, and molar ratio of mineralizer (i.e.S_(m)=S_(m)(T, p, x)). Small changes of S_(m) can be expressed as:

ΔS _(m)≈(∂S _(m) /∂T)|_(p,x) ΔT+(∂S _(m) /∂p)|_(T,x) Δp+(∂S _(m)/∂x)|_(T,p) Δx,

[0088] where the partial differentials (e.g. (∂S_(m)/∂T)|_(p,x))determine the behavior of S_(m) with variation of its parameters (e.g.T). In this specification the partial differentials are called“coefficients” (e.g. (∂S_(m)/∂T)|_(p,x) is a “temperature coefficient ofsolubility” or “temperature coefficient”).

[0089] The diagram shown in FIG. 1 illustrates that the solubilityincreases with pressure and decreases with temperature, which means thatit possesses a negative temperature coefficient and a positive pressurecoefficient. Such features allow obtaining a bulk monocrystallinegallium-containing nitride by dissolution in the higher solubilityconditions, and crystallization in the lower solubility conditions. Inparticular, the negative temperature coefficient means that, in thepresence of a temperature gradient, the chemical transport of gallium ina soluble form can take place from the dissolution zone having a lowertemperature to the crystallization zone having a higher temperature.

[0090] The process according to invention allows the growth of bulkmonocrystalline gallium-containing nitride crystals on thecrystallization seed and leads in particular to the formation ofstoichiometric gallium-containing nitride, obtained in the form of abulk monocrystalline layer grown on a gallium-containing nitridecrystallization seed. Since such a monocrystal is obtained in asupercritical solution that contains ions of alkali metals, it cancontain alkali metals in a quantity higher than 0.1 ppm. Because it isdesired to maintain a purely basic character of the supercriticalsolution, mainly in order to avoid corrosion of the apparatus, halidesare preferably not intentionally introduced into the solvent. Theprocess of the invention can also provide a bulk monocrystallinegallium-containing nitride crystal in which part of the gallium, e.g.from 5 to 50 mol-% may be substituted by Al and/or In. Moreover, thebulk monocrystalline gallium-containing nitride crystal may be dopedwith donor and/or acceptor and/or magnetic dopants. These dopants canmodify optical, electric and magnetic properties of thegallium-containing nitride crystal. With respect to the other physicalproperties, the bulk monocrystalline gallium-containing nitride crystalcan have a dislocation density below 10⁶/cm², preferably below 10⁵/cm²;or most preferably below 10⁴/cm². Besides, the FWHM of the X-ray rockingcurve from (0002) plane can be below 600 arcsec, preferably below 300arcsec, and most preferably below 60 arcsec. The best bulkmonocrystalline gallium nitride obtained may have a dislocation densitylower than 10⁴/cm² and simultaneously a FWHM of the X-ray rocking curvefrom (0002) plane below 60 arcsec.

[0091] Due to their good crystalline quality the gallium-containingnitride crystals obtained in the present invention may be used as asubstrate material for optoelectronic semiconductor devices based onnitrides, in particular for laser diodes.

[0092] The following examples are intended to illustrate the inventionand should not be construed as being limiting.

EXAMPLES

[0093] The dislocation density can be measured by th eso-called EPDmethod (Etch Pit Density) and subsequent evaluation using a microscope

[0094] The FWHM of the X-ray rocking curve can be determined by X-raydiffraction analysis.

[0095] Since it is not possible to readily measure the temperature in anautoclave while in use under supercritical conditions, the temperaturein the autoclave was estimated by the following method. The outside ofthe autoclave is equipped with thermocouples near the dissolution zoneand the crystallization zone. For the calibration, additionalthermocouples were introduced into the inside of the empty autoclave inthe dissolution zone and the crystallization zone. The empty autoclavewas then heated stepwise to various temperatures and the values of thetemperature of the thermocouples inside the autoclave and outside theautoclave were measured and tabulated. For example, if the temperatureof the crystallization zone is determined to be 500° C. and thetemperature of the dissolution zone is 400° C. inside the emptyautoclave when the temperature measured by the outside thermocouples are480° C. and 395° C., respectively. It is assumed that undersupercritical conditions the temperatures in thecrystallization/dissolution zones will also be 500° C./400° C. whentemperatures of 480° C./395° C. are measured by the outsidethermocouples. In reality, the temperature difference between the twozones can be lower due to effective heat transfer through thesupercritical solution.

Example 1

[0096] Two crucibles were placed into a high-pressure autoclave having avolume of 10.9 cm³. The autoclave was manufactured according to a knowndesign [H. Jacobs, D. Schmidt, Current Topics in Materials Science, vol.8, ed. E. Kaldis (North-Holland, Amsterdam, 1981); 381]. One of thecrucibles contained 0.4 g of gallium nitride in the form of 0.1 mm thickplates produced by the HVPE method as feedstock, while the othercontained a gallium nitride seed of a double thickness weighing 0.1 g.The seed was also obtained by the HVPE method. Further, 0.72 g ofmetallic potassium of 4N purity was placed in the autoclave, theautoclave was filled with 4.81 g of ammonia and then closed. Theautoclave was put into a furnace and heated to a temperature of 400° C.The pressure within the autoclave was 2 kbar. After 8 days thetemperature was increased to 500° C., while the pressure was maintainedat the 2 kbar level and the autoclave was maintained under theseconditions for another 8 days.(FIG. 2). As a result of this process, inwhich the dissolution and crystallization steps were separated in time,the feedstock was completely dissolved and the recrystallization ofgallium nitride in the form of a layer took place on the partiallydissolved seed. The two-sided monocrystalline layers had a totalthickness of about 0.4 mm.

Example 2

[0097] Two crucibles were put into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. One of the crucibles contained0.44 g of gallium nitride in the form of 0.1 mm thick plates produced bythe HVPE method as feedstock, and the other contained a gallium nitrideseed of a double thickness weighing 0.1 g, also obtained by the HVPEmethod. Further, 0.82 g of metallic potassium of 4N purity was placed inthe autoclave, the autoclave was filled with 5.43 g of ammonia and thenclosed. The autoclave was put into a furnace and heated to a temperatureof 500° C. The pressure within the autoclave was 3.5 kbar. After 2 daysthe pressure was lowered to 2 kbar, while the temperature was maintainedat the 500° C. level and the autoclave was maintained under theseconditions for another 4 days (FIG. 3). As a result of this process, thefeedstock was completely dissolved and the recrystallization of galliumnitride took place on the partially dissolved seed. The two-sidedmonocrystalline layers had a total thickness of about 0.25 mm.

Example 3

[0098] Two crucibles were placed into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. One of the crucibles contained0.3 g of the feedstock in the form of metallic gallium of 6N purity andthe other contained a 0.1 g gallium nitride seed obtained by the HVPEmethod. Further, 0.6 g of metallic potassium of 4N purity was placed inthe autoclave; the autoclave was filled with 4 g of ammonia and thenclosed. The autoclave was put into a furnace and heated to a temperatureof 200° C. After 2 days the temperature was increased to 500° C., whilethe pressure was maintained at the 2 kbar level and the autoclave wasmaintained in these conditions for further 4 days (FIG. 4). As a resultof this process, the feedstock was completely dissolved and thecrystallization of gallium nitride took place on the seed. The two-sidedmonocrystalline layers had a total thickness of about 0.3 mm.

Example 4

[0099] This is an example of a process, in which the dissolution andcrystallization steps take place simultaneously (recrystallizationprocess). In this example and all the following an apparatus is usedwhich is schematically shown in FIG. 9 and FIG. 10. The basic unit ofthe apparatus is the autoclave 1, which in this Example has a volume of35.6 cm³. The autoclave 1 is equipped with an separating device 2 whichallows for chemical transport of the solvent in the supercriticalsolution inside the autoclave 1. For this purpose, the autoclave 1 isput into a chamber 3 of a set of two furnaces 4 provided with heatingdevices 5 and a cooling device 6. The autoclave 1 is secured in adesired position with respect to the furnaces 4 by means of a screw-typeblocking device 7. The furnaces 4 are mounted on a bed 8 and are securedby means of steel tapes 9 wrapped around the furnaces 4 and the bed 8.The bed 8 together with the set of furnaces 4 is rotationally mounted inbase 10 and is secured in a desired angular position by means of a pininterlock 11. In the autoclave 1, placed in the set of furnaces 4, theconvective flow of supercritical solution takes place as determined bythe separating device 2. The separating device 2 is in the form of ahorizontal baffle 12 having a circumferential opening. The baffle 12separates the dissolution zone 13 from the crystallization zone 14 inthe autoclave 1, and enables, together with the adjustable tilting angleof the autoclave 1, controlling of speed and type of convective flow.The temperature level of the individual zones in the autoclave 1 iscontrolled by means of a control system 15 operating the furnaces 4. Inthe autoclave 1, the dissolution zone 13 coincides with thelow-temperature zone of the set of furnaces 4 and is located above thehorizontal baffle 12 and the feedstock 16 is put into this zone 13. Onthe other hand, the crystallization zone 14 coincides with thehigh-temperature zone of the set of furnaces 4 and it is located belowthe horizontal baffle 12. The crystallization seed 17 is mounted in thiszone 14. The mounting location of the crystallization seed 17 is belowthe intersection of the rising and descending convective streams.

[0100] An amount of 3.0 g of gallium nitride produced by the HVPE methodwas placed in the high-pressure autoclave described above, which was setin the horizontal position. This gallium nitride had the form of platesof about 0.2 mm thickness, and it was distributed (roughly uniformly) inequal portions in the dissolution zone 13 and the crystallization zone14. The portion placed in the dissolution zone 13 played the role offeedstock, whereas the portion placed in the crystallization zone 14played the role of crystallization seeds. Metallic potassium of 4Npurity was also added in a quantity of 2.4 g. Then the autoclave 1 wasfilled with 15.9 g of ammonia (5N); closed, put into a set of furnaces 4and heated to a temperature of 450° C. The pressure inside the autoclave1 was approx. 2 kbar. During this stage, which lasted one day, partialdissolution of gallium nitride was carried out in both zones. Then thetemperature of the crystallization zone 14 was increased to 500° C.while the temperature of the dissolution zone 13 was lowered to 400° C.and the autoclave 1 was kept in these conditions for 6 more days (FIG.5). As a final result of this process, partial dissolution of thefeedstock in the dissolution zone 13 and crystallization of galliumnitride on the gallium nitride seeds in the crystallization zone 14 tookplace.

Example 5

[0101] The above-mentioned high pressure autoclave 1 having a volume of35.6 cm³ was charged with feedstock in the form of a 3.0 g pellet ofsintered gallium nitride (introduced into the dissolution zone 13), twoseeds of gallium nitride obtained by the HVPE method and having the formof plates having a thickness of 0.4 mm and a total weight of 0.1 g(introduced into the crystallization zone 14), as well as with 2.4 g ofmetallic potassium of 4N purity. Then the autoclave was filled with 15.9g of ammonia (5N) and closed. The autoclave 1 was then put into a set offurnaces 4 and heated to 450° C. The pressure inside the autoclave wasabout 2 kbar. After an entire day the temperature of the crystallizationzone 14 was raised to 480° C., while the temperature of the dissolutionzone 13 was lowered to 420° C. and the autoclave was maintained underthese conditions for 6 more days (see FIG. 6). As a result of theprocess the feedstock was partially dissolved in the dissolution zone 13and gallium nitride crystallized on the seeds in the crystallizationzone 14. The two-sided monocrystalline layers had a total thickness ofabout 0.2 mm.

Example 6

[0102] The above-mentioned high pressure autoclave 1 having a volume of35.6 cm³ (see FIG. 9) was charged with 1.6 g of feedstock in the form ofgallium nitride produced by the HVPE method and having the form ofplates having a thickness of about 0.2 mm (introduced into thedissolution zone 13), three gallium-nitride seeds of a thickness ofabout 0.35 mm and a total weight of 0.8 g, also obtained by the HVPEmethod (introduced into the crystallization zone 14), as well as with3.56 g of metallic potassium of 4N purity. The autoclave 1 was filledwith 14.5 g of ammonia (5N) and closed. Then the autoclave 1 was putinto a set of furnaces 4 and heated to 425° C. The pressure inside theautoclave was approx. 1.5 kbar. After an entire day the temperature ofthe dissolution zone 13 was lowered to 400° C. while the temperature ofthe crystallization zone 14 was increased to 450° C. and the autoclavewas kept in these conditions for 8 more days (see FIG. 7). After theprocess, the feedstock was found to be partially dissolved in thedissolution zone 13 and gallium nitride had crystallized on the seeds ofthe HVPE GaN in the crystallization zone 14. The two-sidedmonocrystalline layers had a total thickness of about 0.15 mm.

Example 7

[0103] The above-mentioned high pressure autoclave 1 having a volume of35.6 cm³ (see FIG. 9) was charged in its dissolution zone 13 with 2 g offeedstock in the form of gallium nitride produced by the HVPE method andhaving the form of plates having a thickness of about 0.2 mm. and 0.47 gof metallic potassium of 4N purity, and in its crystallization zone 14with three GaN seeds of a thickness of about 0.3 mm and a total weightof about 0.3 g also obtained by the HVPE method. The autoclave wasfilled with 16.5 g of ammonia (5N) and closed. Then the autoclave 1 wasput into a set of furnaces 4 and heated to 500° C. The pressure insidethe autoclave was approx. 3 kbar. After an entire day the temperature inthe dissolution zone 13 was reduced to 450° C. while the temperature inthe crystallization zone 14 was raised to 550° C. and the autoclave waskept under these conditions for the next 8 days (see FIG. 8). After theprocess, the feedstock was found to be partially dissolved in thedissolution zone 13 and gallium nitride had crystallized on the seeds inthe crystallization zone 14. The two-sided monocrystalline layers had atotal thickness of about 0.4 mm.

Example 8

[0104] An amount of 1.0 g of gallium nitride produced by the HVPE methodwas put into the dissolution zone 13 of the high-pressure autoclave 1having a volume of 35.6 cm³. In the crystallization zone 14 of theautoclave, a crystallization seed of gallium nitride having a thicknessof 100 μm and a surface area of 2.5 cm², obtained by the HVPE method,was placed. Then the autoclave was charged with 1.2 g of metallicgallium of 6N purity and 2.2 g of metallic potassium of 4N purity.Subsequently, the autoclave 1 was filled with 15.9 g of ammonia (5N),closed, put into a set of furnaces 4 and heated to a temperature of 200°C. After 3 days during which period metallic gallium was dissolved inthe supercritical solution—the temperature was increased to 450° C.which resulted in a pressure of about 2.3 kbar. The next day thecrystallization zone temperature was increased to 500° C. while thetemperature of the dissolution zone 13 was lowered to 370° C. and theautoclave 1 was kept in these conditions for the next 20 days (see FIG.11). As a result of this process, the partial dissolution of thematerial in the dissolution zone 13 and the growth of the galliumnitride on the gallium nitride seed in the crystallization zone 14 tookplace. The resulting crystal of gallium nitride having a total thicknessof 350 μm was obtained in the form of two-sided monocrystalline layers.

Example 9

[0105] An amount of 3.0 g of gallium nitride in the form of a sinteredgallium nitride pellet was put into the dissolution zone 13 ofhigh-pressure autoclave 1 having a volume of 35.6 cm³ (see FIG. 9). Inthe crystallization zone 14 of the autoclave, a crystallization seed ofgallium nitride obtained by the HVPE method and having a thickness of120 μm and a surface area of 2.2 cm² was placed. Then the autoclave wascharged with 2.3 g of metallic potassium of 4N purity. Subsequently, theautoclave 1 was filled with 15.9 g of ammonia (5N), closed, put into aset of furnaces 4 and heated to a temperature of 250° C. in order topartially dissolve the sintered GaN pellet and obtain a preliminarysaturation of a supercritical solution with gallium in a soluble form.After two days, the temperature of the crystallization zone 14 wasincreased to 500° C. while the temperature of the dissolution zone 13was lowered to 420° C. and the autoclave 1 was kept in these conditionsfor the next 20 days (see FIG. 12). As a result of this process, partialdissolution of the material in the dissolution zone 13 and growth ofgallium nitride on the gallium nitride seed took place in thecrystallization zone 14. A crystal of gallium nitride having a totalthickness of 500 μm was obtained in the form of two-sidedmonocrystalline layers.

Example 10

[0106] An amount of 0.5 g of gallium nitride plates having an averagethickness of about 120 μm, produced by the HVPE method, were put intothe dissolution zone 13 of high-pressure autoclave 1 having a volume of35.6 cm³. In the crystallization zone 14 of the autoclave, threecrystallization seeds of gallium nitride obtained by the HVPE methodwere placed. The crystallization seeds had a thickness of about 120 μmand a total surface area of 1.5 cm². Then the autoclave was charged with0.41 g of metallic lithium of 3N purity. Subsequently, the autoclave 1was filled with 14.4 g of ammonia (5N), closed, put into a set offurnaces 4 and heated so that the temperature of the crystallizationzone 14 was increased to 550° C. and the temperature of the dissolutionzone 13 was increased to 450° C. The resulting pressure was about 2.6kbar. The autoclave 1 was kept in these conditions for the next 8 days(see FIG. 13). As a result of this process, partial dissolution of thematerial in the dissolution zone 13 and growth of gallium nitride on thegallium nitride seeds in the crystallization zone 14 took place. Theresulting crystals of gallium nitride had a thickness of 40 μm and werein the form of two-sided monocrystalline layers.

Example 11

[0107] An amount of 0.5 g of gallium nitride having an average thicknessof about 120 μm, produced by the HVPE method, was placed into thedissolution zone 13 of hill-pressure autoclave 1 having a volume of 35.6cm³. In the crystallization zone 14 of the autoclave, threecrystallization seeds of gallium nitride obtained by the HVPE methodwere placed. The crystallization seeds had a thickness of 120 μm and atotal surface area of 1.5 cm². Then the autoclave was charged with 0.071g of metallic gallium of 6N purity and 1.4 g of metallic sodium of 3Npurity. Subsequently, the autoclave 1 was filled with 14.5 g of ammonia(5N), closed, put into a set of furnaces 4 and heated to a temperatureof 200° C. After 1 day—during which period metallic gallium wasdissolved in the supercritical solution—the autoclave 1 was heated sothat the temperature in the crystallization zone was increased to 500°C., while the temperature in the dissolution zone was increased to 400°C. The resulting pressure was about 2.3 kbar. The autoclave 1 was keptin these conditions for the next 8 days (see FIG. 14). As a result ofthis process, partial dissolution of the material in the dissolutionzone 13 and growth of gallium nitride on the gallium nitride seeds inthe crystallization zone 14 took place. The resulting crystals ofgallium nitride were obtained in the form of two-sided monocrystallinelayers having a total thickness of 400 μm.

Example 12

[0108] An amount of 0.5 g of gallium nitride having an average thicknessof about 120 μm, produced by the HVPE method, was placed into thedissolution zone 13 of the high-pressure autoclave 1 having a volume of35.6 cm³. In the crystallization zone 14 of the autoclave, threecrystallization seeds of gallium nitride obtained by the HVPE methodwere placed. The crystallization seeds had a thickness of 120 μm and atotal surface area of 1.5 cm². Then the autoclave was charged with 0.20g of gallium amide and 1.4 g of metallic sodium of 3N purity.Subsequently, the autoclave 1 was filled with 14.6 g of ammonia (5N),closed, put into a set of furnaces 4 and heated to a temperature of 200°C. After 1 day—during which period gallium amide was dissolved in thesupercritical solution—the autoclave 1 was heated so that thetemperature in the crystallization zone was increased to 500° C., whilethe temperature in the dissolution zone was increased to 400° C. Theresulting pressure was about 2.3 kbar. The autoclave 1 was kept in theseconditions for the next 8 days (see also FIG. 14). As a result of thisprocess, partial dissolution of the material in the dissolution zone 13and growth of gallium nitride on the gallium nitride seeds in thecrystallization zone 14 took place. The resulting crystals of galliumnitride were in the form of two-sided monocrystalline layers having atotal thickness of 490 μm.

Example 13

[0109] One crucible was placed into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. The crucible contained 0.3 g ofthe feedstock in the form of metallic gallium of 6N purity. Also threegallium-nitride seeds having a thickness of about 0.5 mm and a totalmass of 0.2 g, all obtained by the HVPE method, were suspended withinthe autoclave. Further, 0.5 g of metallic sodium of 3N purity was placedin the autoclave; the autoclave was filled with 5.9 g of ammonia andthen closed. The autoclave was put into a furnace and heated to atemperature of 200° C., where the pressure was about 2.5 kbar. After 1day the temperature was increased to 500° C., while the pressureincreased up to 5 kbar and the autoclave was maintained in theseconditions for further 2 days (FIG. 16). As a result of this process,the feedstock was completely dissolved and crystallization of galliumnitride took place on the seed. The average thickness of thetwo-side-overgrown monocrystalline layer of gallium nitride was about0.14 mm. The FWHM of the X-ray rocking curve from the (0002) plane atthe gallium-terminated side was 43 arcsec, while at thenitrogen-terminated side it was 927 arcsec.

[0110] The monocrystalline gallium nitride layers have a wurzitestructure like in all of the other examples.

1. A process for obtaining a gallium-containing nitride crystal,comprising the steps of: (i) providing a gallium-containing feedstock,an alkali metal-containing component, at least one crystallization seedand a nitrogen-containing solvent in at least one container; (ii)bringing the nitrogen-containing solvent into a supercritical state;(iii) at least partially dissolving the gallium-containing feedstock ata first temperature and at a first pressure; and (iv) crystallizinggallium-containing nitride on the crystallization seed at a secondtemperature and at a second pressure while the nitrogen-containingsolvent is in the supercritical state; wherein at least one of thefollowing criteria is fulfilled: (a) the second temperature is higherthan the first temperature; and (b) the second pressure is lower thanthe first pressure.
 2. The process according to claim 1, wherein the atleast one container is an autoclave.
 3. The process according to claim1, wherein the gallium-containing feedstock is at least partiallydissolved before step (iv).
 4. The process according to claim 1, whereinthe gallium-containing feedstock is at least partially dissolved duringstep (iv).
 5. The process according to claim 1, wherein the process isconducted in a container having a dissolution zone at the firsttemperature and a crystallization zone at the second temperature andwherein the second temperature is higher than the first temperature. 6.The process according to claim 5, wherein the difference in temperaturebetween the dissolution zone and crystallization zone is selected so asto ensure convective transport in the supercritical solution.
 7. Theprocess according to claim 6, wherein the difference in temperaturebetween the second temperature and the first temperature is at least 1°C.
 8. The process according to claim 7, wherein the difference intemperature between the second temperature and the first temperature isfrom about 5 to about 150° C.
 9. The process according to claim 1,wherein the gallium-containing nitride has the general formulaAl_(x)Ga_(1-x-y)In_(y)N, where 0≦x<1, 0≦y<1, and 0≦x+y<1.
 10. Theprocess according to claim 9, wherein the gallium-containing nitride hasthe general formula Al_(x)Ga_(1-x-y)In_(y)N, where 0≦x<0.5 and 0≦y<0.5.11. The process according to claim 9, wherein the gallium-containingnitride is gallium nitride.
 12. The process according to claim 1,wherein the gallium-containing nitride further contains at least onedonor dopant, at least one acceptor dopant, at least one magnetic dopantor mixtures thereof.
 13. The process according to claim 1, wherein thegallium-containing feedstock comprises at least one compound selectedfrom the group consisting of gallium nitride, gallium azides, galliumimides, gallium amido-imides, gallium hydrides, gallium-containingalloys, metallic gallium and mixtures thereof.
 14. The process accordingto claim 13, wherein the gallium-containing feedstock comprises metallicgallium and gallium nitride.
 15. The process according to claim 13,wherein the feedstock further comprises an aluminium feedstock, anindium feedstock or mixtures thereof, wherein the feedstocks areselected from the group consisting of nitrides, azides, imides,amido-imides, hydrides, alloys, metallic aluminium, and metallic indium.16. The process according to claim 1, wherein the alkalimetal-containing component is at least one metallic alkali metal or atleast one alkali metal salt.
 17. The process according to claim 16,wherein the alkali metal in the alkali metal-containing component islithium, sodium, potassium or cesium.
 18. The process according to claim17, wherein the alkali metal in the alkali metal-containing component issodium or potassium.
 19. The process according to claim 16, wherein thealkali metal salt is an amide, an imide or an azide.
 20. The processaccording to claim 1, wherein a surface of the at least onecrystallization seed is a crystalline layer of a gallium-containingnitride.
 21. The process according to claim 20, wherein thegallium-containing nitride of the crystalline layer has the generalformula Al_(x)Ga_(1-x-y)In_(y)N, where 0≦x<1, 0≦y<1, and 0≦x+y<1. 22.The process according to claim 20, wherein the crystalline layer has adislocation density of less than 10⁶/cm².
 23. The process according toclaim 1, wherein the nitrogen-containing solvent is ammonia, aderivative thereof, or mixtures thereof.
 24. The process according toclaim 1, wherein the first temperature and the second temperature arefrom about 100° C. to about 800° C. and wherein the second temperatureis at least 1° C. higher than the first temperature.
 25. The processaccording to claim 24, wherein the first temperature and the secondtemperature are from about 300° C. to about 600° C.
 26. The processaccording to claim 25, wherein the first temperature and the secondtemperature are from about 400° C. to about 550° C.
 27. The processaccording to claim 1, wherein the first pressure and the second pressureare the same and are from about 1000 bar (10⁵ kPa) to about 10 000 bar(10⁶ kPa).
 28. The process according to claim 27, wherein the firstpressure and the second pressure are the same and are from about 1000bar (10⁵ kPa) to about 5500 bar (5.5×10⁵ kPa).
 29. The process accordingto claim 28, wherein the first pressure and the second pressure are thesame and are from about 1500 bar (1.5×10⁵ kPa) to about 3000 bar (3×10⁵kPa).
 30. The process according to claim 1, wherein step (iv) isconducted so that the crystallization selectively takes place on thecrystallization seed.
 31. A process for preparing a gallium-containingnitride crystal comprising the steps of: (i) providing agallium-containing feedstock comprising at least two differentcomponents, an alkali metal-containing component, at least onecrystallization seed and a nitrogen-containing solvent in a containerhaving a dissolution zone and a crystallization zone, whereby thegallium-containing feedstock is provided in the dissolution zone and theat least one crystallization seed is provided in the crystallizationzone; (ii) subsequently bringing the nitrogen-containing solvent into asupercritical state; (iii) subsequently partially dissolving thegallium-containing feedstock at a dissolution temperature and at adissolution pressure in the dissolution zone, whereby a first componentof the gallium-containing feedstock is substantially completelydissolved and a second component of the gallium-containing feedstock aswell as the crystallization seed(s) remain substantially undissolved sothat an undersaturated or saturated solution with respect togallium-containing nitride is obtained; (iv) subsequently setting theconditions in the crystallization zone at a second temperature and at asecond pressure so that over-saturation with respect togallium-containing nitride is obtained and crystallization ofgallium-containing nitride occurs on the at least one crystallizationseed and setting the conditions in the dissolution zone at a firsttemperature and at a first pressure so that the second component of thegallium-containing feedstock is dissolved; wherein the secondtemperature is higher than the first temperature.
 32. The processaccording to claim 31, wherein the first component of thegallium-containing feedstock is metallic gallium and the secondcomponent of the gallium-containing feedstock is gallium nitride. 33.The process according to claim 31, wherein the crystallization isconducted so that it selectively takes place on the crystallizationseed.
 34. The process according to claim 31, wherein the firsttemperature and the first pressure in the dissolution zone and thesecond temperature and the second pressure in the crystallization zoneare selected so that the concentration of gallium in the over-saturatedsolution remains substantially the same during crystallization.
 35. Theprocess according to claim 31, wherein the container comprises at leastone baffle between the dissolution zone and the crystallization zone.36. The process according to claim 35, wherein the at least one bafflehas a central opening, circumferential openings or a combinationthereof.
 37. A gallium-containing nitride crystal obtainable by aprocess according to any one of claims 1 to
 36. 38. A gallium-containingnitride crystal having a surface area of more than 2 cm² and having adislocation density of less than 10⁶/cm².
 39. A gallium-containingnitride crystal having a thickness of at least 200 μm and a full widthat half maximum (FWHM) of X-ray rocking curve from (0002) plane of 50arcsec or less.
 40. The gallium-containing nitride crystal according toclaim 39 wherein the thickness is at least 500 μm.
 41. Thegallium-containing nitride crystal according to any of claims 37 to 40,wherein the gallium-containing nitride crystal has the general formulaAl_(x)Ga_(1-x-y)In_(y)N, where 0≦x<1, 0≦y<1, and 0≦x+y<1.
 42. Thegallium-containing nitride crystal according to any of claims 37 to 40,wherein the gallium-containing nitride crystal contains alkali elementsin an amount of more than about 0.1 ppm.
 43. The gallium-containingnitride crystal according to any of claims 37 to 40, wherein thegallium-containing nitride crystal has a halogen content of about 0.1ppm or less.
 44. The gallium-containing nitride crystal according to anyof claims 37 to 40, wherein the gallium-containing nitride crystal has avolume of more than 0.05 cm³.
 45. The gallium-containing nitride crystalaccording to any of claims 37 to 40, wherein the gallium-containingnitride crystal contains at least one element selected from the groupconsisting of Ti, Fe, Co, Cr, and Ni.
 46. The gallium-containing nitridecrystal according to any of claims 37 to 40, wherein thegallium-containing nitride crystal additionally contains at least onedonor dopant and/or at least one acceptor dopant and/or at least onemagnetic dopant in a concentration from 10¹⁷ to 10²¹/cm³.
 47. Thegallium-containing nitride crystal according to any of claims 37 to 40,wherein the layer of gallium-containing nitride crystal further containsAl and/or In and the molar ratio of Ga to Al and/or In is more than 0.5.48. The gallium-containing nitride crystal according to any of claims 37to 40, wherein the gallium-containing nitride crystal contains a seed.49. The gallium-containing nitride crystal according to any of claims 37to 40, wherein the gallium-containing nitride crystal ismonocrystalline.
 50. An apparatus for obtaining a gallium-containingnitride crystal comprising an autoclave 1 having an internal space andcomprising at least one device 4, 5 for heating the autoclave to atleast two zones having different temperatures, wherein the autoclavecomprises a device which separates the internal space into a dissolutionzone 13 and a crystallization zone
 14. 51. The apparatus according toclaim 50, wherein the at least one device is for heating the autoclaveto two zones having different temperatures and the two zones coincidewith the dissolution zone 13 and the crystallization zone
 14. 52. Theapparatus according to claim 50, wherein the device which separates theinternal space is at least one baffle 12 having at least one opening.53. The apparatus according to claim 52, wherein the at least one baffle12 has a central opening, circumferential openings or a combinationthereof.
 54. The apparatus according to claim 50, wherein thecrystallization zone 14 is provided with a heating device 5 for heatingthe crystallization zone 14 to a temperature higher than the temperatureof the dissolution zone
 13. 55. The apparatus according to claim 50,wherein a seed-holder 18 is provided in the crystallization zone 14 anda feedstock-holder 19 is provided in the dissolution zone
 13. 56. Theapparatus according to claim 52, wherein the baffle(s) is/are in ahorizontal position and wherein the dissolution zone 13 is located abovesaid horizontal baffle or horizontal baffles 12, whereas saidcrystallization zone 14 is located below said horizontal baffle orhorizontal baffles
 12. 57. A process for obtaining a bulkmonocrystalline gallium-containing nitride crystal, wherein it isperformed in an autoclave, in the environment of a supercritical solventcontaining ions of alkali metals, wherein a gallium-containing feedstockfor making said gallium-containing nitride crystal becomes dissolved insaid supercritical solvent to form a supercritical solution, and thegallium-containing nitride becomes crystallized from the supercriticalsolution on the surface of a crystallization seed at a temperaturehigher and/or pressure lower than that of the feedstock dissolution inthe supercritical solvent.
 58. The process according to claim 57,wherein said process comprises the steps of dissolving thegallium-containing feedstock and a separate step of transferring thesupercritical solution to the higher temperature and/or to the lowerpressure.
 59. The process according to claim 57, wherein said processcomprises the step of simultaneous creation of at least two zones ofdifferent temperatures, said gallium-containing feedstock is placed inthe dissolution zone of the lower temperature, while the crystallizationseed is placed in the crystallization zone of the higher temperature.60. The process according to claim 59, wherein said temperaturedifference between said dissolution zone and said crystallization zoneis controlled so as to ensure chemical transport in the supercriticalsolution.
 61. The process according to claim 60, wherein said chemicaltransport in the supercritical solution takes place through convectionin the autoclave.
 62. The process according to claim 60, wherein saidtemperature difference between the dissolution zone and thecrystallization zone is greater than 1° C.
 63. The process according toclaim 57, wherein said gallium-containing nitride crystal has theformula Al_(x)Ga_(1-x-y)In_(y)N, where 0≦x<1, 0≦y<1, 0≦x+y<1.
 64. Theprocess according to claim 57, wherein said gallium-containing nitridecrystal contains dopants of a donor and/or acceptor and/or magnetictype.
 65. The process according to claim 57, wherein said supercriticalsolvent contains NH₃ and/or its derivatives.
 66. The process accordingto claim 57, wherein said supercritical solvent contains sodium and/orpotassium ions.
 67. The process according to claim 57, wherein saidgallium-containing feedstock consists essentially of gallium-containingnitride and/or its precursors.
 68. The process according to claim 67,wherein said precursors are selected from the group consisting ofgallium azides, gallium imides, gallium amido-imides, gallium amides,gallium hydrides, gallium-containing alloys, and metallic gallium andoptionally corresponding compounds of other elements of Group XIII(according to IUPAC, 1989).
 69. The process according to claim 57,wherein said crystallization seed has at least a crystalline layer ofgallium-containing nitride.
 70. The process according to claim 57,wherein said crystallization seed has at least a crystalline layer ofgallium-containing nitride with a dislocation density below 10⁶/cm². 71.The process according to claim 57, wherein said crystallization of agallium-containing nitride takes place at a temperature from 100 to 800°C., preferably 300 to 600° C., more preferably 400 to 550° C.
 72. Theprocess according to claim 57, wherein said crystallization of agallium-containing nitride takes place at a pressure from 100 to 10000bar, preferably, 1000 to 5500 bar, more preferably 1500 to 3000 bar. 73.The process according to claim 57, wherein the content of alkali metalions in the supercritical solvent is controlled so as to provideadequate levels of solubility of said feedstock as well as of saidgallium-containing feedstock.
 74. The process according to claim 57,wherein the molar ratio of the moles of said alkali metal ions to themoles of the supercritical solvent is controlled within the range of1:200 to 1:2, preferably 1:100 to 1:5, more preferably 1:20 to 1:8. 75.An apparatus for obtaining of a monocrystalline gallium-containingnitride crystal, comprising an autoclave 1 for producing supercriticalsolvent, equipped with an installation 2 for establishing a convectiveflow, the autoclave being mounted inside a furnace or set of furnaces 4which are equipped with heating devices 5 and/or cooling devices
 6. 76.The apparatus according to claim 75, wherein said furnace or set offurnaces 4 has a high-temperature zone coinciding with thecrystallization zone 14 of said autoclave 1 equipped with heatingdevices 5, and a low-temperature zone coinciding with the dissolutionzone 13 of the autoclave 1 equipped with heating devices 5 and/orcooling devices
 6. 77. The apparatus according to claim 76, wherein saidfurnace or set of furnaces 4 has a high-temperature zone coinciding withthe crystallization zone 14 of said autoclave 1 equipped with heatingdevices 5 and/or cooling devices 6, as well as a low-temperature zonecoinciding with the dissolution zone 13 of the autoclave 1 equipped withheating devices 5 and/or cooling devices
 6. 78. The apparatus accordingto claim 76, wherein said installation 2 is in the form of a horizontalbaffle or horizontal baffles 12 having central and/or circumferentialopenings, separating the crystallization zone 14 from the dissolutionzone
 13. 79. The apparatus according to claim 76, wherein feedstock 16is placed in the autoclave 1 in the dissolution zone 13, and said acrystallization seed 17 is placed in the crystallization zone 14, andsaid convective flow between the zones 13 and 14 is established by saidinstallation
 2. 80. The apparatus according to claim 79, wherein saiddissolution zone 13 is located above said horizontal baffle orhorizontal baffles 12, whereas said crystallization zone 14 is locatedbelow said horizontal baffle or horizontal baffles
 12. 81. A process forpreparing a bulk monocrystalline gallium-containing nitride crystal inan autoclave, which comprises the steps of (i) providing a supercriticalammonia solution containing ions of alkali metal and gallium in asoluble form by introducing a gallium-containing feedstock tosupercritical ammonia solvent containing ions of alkali metals, in whichsolubility of gallium-containing nitride shows a negative temperaturecoefficient in said supercritical ammonia solution, and (ii)crystallizing said gallium-containing nitride selectively on acrystallization seed from said supercritical ammonia solution by meansof the negative temperature coefficient of solubility.
 82. A process forpreparing a bulk monocrystalline gallium-containing nitride in anautoclave, which comprises the steps of (i) providing a supercriticalammonia solution containing ions of alkali metal and gallium in asoluble form by introducing a gallium-containing feedstock into asupercritical ammonia solvent containing ions of alkali metals, in whichsolubility of gallium-containing nitride shows a positive pressurecoefficient in said supercritical ammonia solution, and (ii)crystallizing said gallium-containing nitride selectively on acrystallization seed from said supercritical ammonia solution by meansof the positive pressure coefficient of solubility.
 83. A process forpreparing a bulk monocrystalline gallium-containing nitride in anautoclave according to claim 81 or 82, wherein said gallium-containingnitride is GaN.
 84. A process for preparing a bulk monocrystallinegallium-containing nitride in an autoclave according to claim 81 or 82,wherein said ion of alkali metal is selected from the group consistingof Li⁺, Na⁺, and K⁺.
 85. A process for preparing a bulk monocrystallinegallium-containing nitride in an autoclave according to claim 81 or 82,wherein said ions of alkali metals are introduced in the form ofmineralizers selected from alkali metals and compounds thereof, such asazides, nitrides, amides, amido-imides, imides, and/or hydrides, forforming an ammono-basic supercritical ammonia solution, which does notcontain ions of halogens.
 86. A process for preparing a bulkmonocrystalline gallium-containing nitride in an autoclave according toclaim 81 or 82, wherein said gallium-containing nitride is dissolved insaid supercritical ammonia solvent in the form of gallium complexcompounds containing alkali metals and NH₃ and/or its derivatives.
 87. Aprocess for preparing a bulk monocrystalline gallium-containing nitridein an autoclave according to claim 81 or 82, wherein said galliumcomplex compound in said supercritical ammonia solution is formed from adissolution of GaN and/or dissolution of metallic Ga with supercriticalammonia solvent.
 88. A process for preparing a supercritical ammoniasolution containing gallium-containing nitride, which comprises thesteps of (i) providing a supercritical ammonia solvent by means ofadjusting a temperature and/or pressure in an autoclave and (ii)dissolving a precursor of gallium-containing nitride in saidsupercritical ammonia solvent to form soluble gallium complex compoundsat a temperature lower than that at which dissolving ofgallium-containing nitride takes place effectively.
 89. A process forpreparing a supercritical ammonia solution containing gallium-containingnitride according to claim 88, wherein the step of dissolving saidprecursor in said supercritical ammonia solvent is carried out at atemperature of 150 to 300° C.
 90. A process for controlling therecrystallization of a gallium-containing nitride in a supercriticalammonia solution, which comprises the steps of (i) preparing asupercritical ammonia solution containing soluble gallium complexcompounds formed by dissolving of gallium-containing nitride feedstockin an autoclave and (ii) decreasing the solubility of saidgallium-containing nitride in the supercritical ammonia solution byincreasing the temperature above that at which dissolving of thegallium-containing nitride feedstock is carried out.
 91. A process forcontrolling the recrystallization of a gallium-containing nitride in asupercritical ammonia solution which comprises the steps of preparing asupercritical ammonia solution containing soluble gallium complexcompounds formed by dissolving of gallium-containing nitride feedstockin the dissolution zone and (ii) controlling over-saturation of saidsupercritical ammonia solution with respect to the crystallization seed,while maintaining a temperature in the crystallization zone lower thanthat in the dissolution zone.
 92. A process for controllingrecrystallization of a gallium-containing nitride in a supercriticalammonia solution according to claim 91, in which over-saturation of saidsupercritical solution with respect to said crystallization seed ismaintained below the level where the phenomenon of spontaneousnucleation of gallium-containing nitride appears.
 93. A process forcontrolling recrystallization of a gallium-containing nitride in asupercritical ammonia solution according to claim 91, in whichover-saturation of said supercritical ammonia solution with respect tosaid crystallization seed, is controlled by adjusting pressure andcomposition of the supercritical ammonia solvent.
 94. A process forcontrolling recrystallization of a gallium-containing nitride in asupercritical ammonia solution according to claim 91, in whichover-saturation of said supercritical ammonia solution is controlled byadjusting the crystallization temperature.
 95. A process for controllingrecrystallization of a gallium-containing nitride in a supercriticalammonia solution according to claim 91, in which over-saturation of saidsupercritical ammonia solution is controlled by adjusting thetemperature difference between the dissolution zone and thecrystallization zone.
 96. A process for controlling recrystallization ofa gallium-containing nitride in a supercritical ammonia solutionaccording to claim 91, in which over-saturation of said supercriticalammonia solution is controlled by adjusting the rate of chemicaltransport.
 97. A process for controlling recrystallization of agallium-containing nitride in a supercritical ammonia solution accordingto claim 90, in which controlling solubility of said gallium-containingnitride in the supercritical ammonia solution is carried out byadjusting the convective flow between the dissolution zone and thecrystallization zone.
 98. A process for controlling recrystallization ofa gallium-containing nitride in a supercritical ammonia solutionaccording to claim 90, in which controlling solubility of saidgallium-containing nitride in the supercritical ammonia solution iscarried out by adjusting the opening ratio of a baffle or bafflesbetween the dissolution zone and the crystallization zone.
 99. Substratefor epitaxy crystallized on the surface of a crystallization seed,especially a substrate for nitride semiconductor layers, wherein thesubstrate has a layer of bulk monocrystalline gallium-containingnitride, has a surface area of more than 2 cm² and has a dislocationdensity of less than 10⁶/cm².
 100. Substrate for epitaxy according toclaim 99, wherein the layer of bulk monocrystalline gallium-containingnitride has the general formula Al_(x)Ga_(1-x-y)In_(y)N, where 0≦x<1,0≦y<1, and 0≦x+y<1.
 101. Substrate for epitaxy according to claim 99,wherein the substrate contains alkali elements in an amount of more thanabout 0.1 ppm.
 102. Substrate for epitaxy according to claim 99, whereinthe layer of bulk monocrystalline gallium-containing nitride has ahalogen content that does not exceed about 0.1 ppm.
 103. Substrate forepitaxy according to claim 99, wherein the layer of bulk monocrystallinegallium-containing nitride has volume of more than 0.05 cm³. 104.Substrate for epitaxy according to claim 99, wherein in the layer ofbulk monocrystalline gallium-containing nitride has a full width at halfmaximum (FWHM) of X-ray rocking curve from (0002) plane of less than 600arcsec.
 105. Substrate for epitaxy according to claim 99, wherein thelayer of bulk monocrystalline gallium-containing nitride additionallycontains at least one donor dopant and/or at least one acceptor dopantand/or at least one magnetic dopant in a concentration from 10¹⁷ to10²¹/cm³.
 106. Substrate for epitaxy according to claim 99, wherein thelayer of bulk monocrystalline gallium-containing nitride contains Aland/or In and the molar ratio of Ga to Al and/or In is more than 0.5.107. Substrate for epitaxy according to claim 99, wherein the layer ofbulk monocrystalline gallium-containing nitride is crystallized on thesurface of a crystallization seed of gallium-containing nitride having adislocation density of less than 10⁶/cm².
 108. Substrate for epitaxyaccording to claim 99, wherein the layer of bulk monocrystallinegallium-containing nitride has a dislocation density of less than10⁴/cm² and a full width at half maximum (FWHM) of X-ray rocking curvefrom (0002) plane of less than 60 arcsec.